Cartilage is an avascular tissue of which chondrocytes are the main cellular component. The chondrocytes in normal articular cartilage occupy approximately 5% of the tissue volume, while the extra-cellular matrix makes up the remaining 95% of the tissue. The chondrocytes secrete the components of the matrix, mainly proteoglycans and collagens, which in turn supply the chondrocytes with an environment suitable for their survival under mechanical stress. In cartilage, collagen type II, together with the protein collagen type IX, is arranged in solid fibril-like structures which provide cartilage with great mechanical strength. The proteoglycans can absorb water and are responsible for the resilient and shock absorbing properties of the cartilage.
One of the functional roles of cartilage in the joint is to allow bones to articulate on each other smoothly. Loss of articular cartilage, therefore, causes the bones to rub against each other leading to pain and loss of mobility. The degradation of cartilage can have various causes. In inflammatory arthritides, as rheumatoid arthritis for example, cartilage degradation is caused by the secretion of proteases (e.g. collagenases) by inflamed tissues (the inflamed synovium for example). Cartilage degradation can also be the result of an injury of the cartilage, due to an accident or surgery, or exaggerated loading or ‘wear and tear’. Cartilage degradation may also be the result of an imbalance in cartilage synthesizing (anabolic) and cartilage degrading (catabolic) processes. The ability of cartilage tissue to regenerate after such insults is limited. Chondrocytes in injured cartilage often display reduced anabolic activity and/or increased catabolic activity. The limited ability of cartilage to self-repair after injury, disease, or surgery is a major limiting factor in rehabilitation of degrading joint surfaces and injury to meniscal cartilage.
The degeneration of cartilage is the hallmark of various diseases, among which rheumatoid arthritis and osteoarthritis are the most prominent.
Osteoarthritis (also referred to as OA, or wear-and-tear arthritis) is the most common form of arthritis and is characterized by loss of articular cartilage, often associated with hypertrophy of the bone and pain. The disease mainly affects hands and weight-bearing joints such as knees, hips and spines. This process thins the cartilage. When the surface area has disappeared due to the thinning, a grade I osteoarthritis is reached; when the tangential surface area has disappeared, grade II osteoarthritis is reached. There are further levels of degeneration and destruction, which affect the deep and the calcified cartilage layers that border with the subchondral bone. For an extensive review on osteoarthritis, we refer to Wieland et al., 2005.
Rheumatoid arthritis (RA) is a chronic joint degenerative disease, characterized by inflammation and destruction of the joint structures. When the disease is unchecked, it leads to substantial disability and pain due to loss of joint functionality and even premature death. The aim of an RA therapy, therefore, is not to slow down the disease but to attain remission in order to stop the joint destruction. Besides the severity of the disease outcome, the high prevalence of RA (˜0.8% of the adults are affected worldwide) means a high socio-economic impact. (For reviews on RA, we refer to Smolen and Steiner (2003); Lee and Weinblatt (2001); Choy and Panayi (2001); O'Dell (2004) and Firestein (2003)).
The clinical manifestations of the development of the osteoarthritis condition are: increased volume of the joint, pain, crepitation and functional disability that lead to pain and reduced mobility of the joints. When disease further develops, pain at rest emerges. If the condition persists without correction and/or therapy, the joint is destroyed leading to disability. Replacement surgery with total prosthesis is then required.
In mature articular cartilage, chondrocytes maintain the cartilage-specific matrix phenotype. Early signs of OA include progressive loss from articular cartilage of the proteoglycan aggrecan, due to damage to type II collagen. This protein represents the major structural collagen found in articular cartilage in healthy individuals. There is ordinarily a strict balance between the production of type II collagen and degradation of this protein by catabolic enzymes during normal remodeling of cartilage. Pathological conditions such as OA are characterized by a loss of this balance with increased proteolysis.
In general, elevated expression of MMPs is associated with the degradation of cartilage and/or extracellular matrix (ECM) but not all proteases are capable of degrading native collagen. Among the matrix metallo proteinases, MMP1, MMP8, MMP13 and MMP14 display the highest capacity for degrading collagen type II. Expression and contents of MMP-1 (collagenase-1) and MMP-13 (Mitchel et al., 1996; Shlopov et al., 1997), expression of MMP-8 (collagenase-2), and collagenase activity (Billinghurst et al., 1997, Dahlberg et al., 2000) are upregulated in human OA cartilage. In particular, MMP-13, also known as collagenase-3, is thought to play an important role in type II collagen degradation in articular cartilage and especially in OA (Billinghurst et al., 1997, Mitchell et al., 1996, Dahlberg et al., 2000, Billinghurst et al., 2000) as indicated by various observations. 1) The expression of MMP13 is increased in the cartilage of OA patients and of animals subjected to arthritogenic surgery like meniscectomy (Appleton et al., 2007) 2) The localization of MMP1 and MMP13 in arthritic cartilage appear to coincide with the location of cartilage destruction, as revealed by antibodies revealing neo-epitopes induced by cartilage cleavage. (Wu et al., 2002) 3) Overexpression of MMP13 in cartilage of transgenic mice lead to an OA-like cartilage destruction phenotype (Neuhold et al., 2001). 4) Type II collagen is the preferred substrate for MMP-13 (Billinghurst et al., 1997; Mitchell et al., 1996). Taken together, MMP13 is well-accepted as a key player in OA-induced cartilage and ECM degeneration.
Therapeutic methods for the correction of the articular cartilage lesions that appear during osteoarthritic disease have been developed, but so far none of them have been able to mediate the regeneration of articular cartilage in situ and in vivo.
Osteoarthritis is difficult to treat. At present, no cure is available and treatment focuses on relieving pain and preventing the affected joint from becoming deformed. Common treatments include the use of non-steroidal anti-inflammatory drugs (NSAIDs). Although dietary supplements such as chondroitin and glucosamine sulphate have been advocated as safe and effective options for the treatment or amelioration of osteoarthritis, a recent clinical trial revealed that both treatments did not reduce pain associated with osteoarthritis (Clegg et al., 2006). Taken together, no disease modifying osteoarthritic drugs are available.
In severe cases, joint replacement may be necessary. This is especially true for hips and knees. If a joint is extremely painful and cannot be replaced, it may be fused. This procedure stops the pain, but results in the permanent loss of joint function, making walking and bending difficult.
Another possible treatment is the transplantation of cultured autologous chondrocytes. Here, chondral cellular material is taken from the patient, sent to a laboratory where it is expanded. The material is then implanted in the damaged tissues to cover the tissue's defects.
Another treatment includes the intra-articular instillation of Hylan G-F 20 (e.g. Synvisc®, Hyalgan®, Artz®), a substance that improves temporarily the rheology of the synovial fluid, producing an almost immediate sensation of free movement and a marked reduction of pain.
Other reported methods include application of tendinous, periosteal, fascial, muscular or perichondral grafts; implantation of fibrin or cultured chondrocytes; implantation of synthetic matrices, such as collagen, carbon fiber; administration of electromagnetic fields. All of these have reported minimal and incomplete effects, resulting in a poor quality tissue that can neither support the weighted load nor allow the restoration of an articular function with normal movement.
Stimulation of the anabolic processes, blocking catabolic processes, or a combination of these two, may result in stabilization of the cartilage, and perhaps even reversion of the damage, and therefore prevent further progression of the disease.
The present invention relates to the relationship between the function of selected proteins identified by the present inventors (hereinafter referred to as “TARGETS”) and inhibition of cartilage and/or extra-cellular matrix (ECM) degradation and inhibition of inflammation.